The present invention relates to a developing apparatus for developing an electrostatic latent image formed on an image bearing member. In particular, it relates to a developing apparatus which is employed in a copying machine, a printer, a facsimile machine, or the like.
In an image forming apparatus such as an electrophotographic copying machine, an electrophotographic printer, and the like, an electrostatic latent image formed on a photoconductive member by exposing the photoconductive member to an optical image in accordance with an intended image, is visualized, that is, developed into a visual image, by adhering the developer borne on a developer bearing member to the latent image on the photoconductive member, by forming an electric field in a development station in which the portion of the photoconductive member, across which the latent image is borne, and the portion of the developer bearing member, across which developer is borne, oppose each other.
As for a developing method, a jumping developing method (Japanese Patent Application Publication No. 58-32375) is widely known. In a jumping developing method, an electrostatic latent image is developed by applying development bias, which is a combination of a DC voltage and a AC voltage having a rectangular waveform, to a development sleeve (developer bearing member) disposed in a manner to hold a predetermined gap relative to the photoconductive member. As the development bias is applied to the development sleeve, the developer particles are oscillated, by the alternating component of the development bias, in the predetermined gap between the development sleeve and the photoconductive member, in the direction virtually in parallel with the line connecting the centers of the axes of the development sleeve and photoconductive member. As a result, the developer particles on the photoconductive member are adhered to the electrostatic latent image on the photoconductive member; in other words, the latent image is developed.
In order to allow a user to obtain an image preferable to the user, a substantial number of image forming apparatuses which employ electrophotographic technologies are provided with a density adjusting apparatus. In theme image forming apparatuses, the amount of developer adhesion is adjusted by adjusting the development contrast, that is, the difference between the potential level of the image portions of the electrostatic latent image on the photoconductive member, and the potential level of the DC component of the development bias, by adjusting the DC component of the development bias.
In some of the known methods for adjusting the density by controlling the bias voltage, the magnitude of the DC voltage, which is applied to a development sleeve in combination with AC voltage having a rectangular waveform, is changed. For example, in order to increase the density, the amount by which the developer particles adhere to the image bearing member is increased by raising the DC voltage to increase transfer voltage (component of the development voltage which induces developer particles to jump, or transfer, from the development sleeve to the photoconductive member).
In other known types of the density adjusting methods, the development density is changed by changing the ratio of the duration of the reverse transfer voltage (component which induces developer particles to transfer back from the photoconductive member to the development sleeve) relative to the duration of the transfer voltage, instead of varying the magnitudes of the transferring and/or reversely transferring components of the bias voltage. For example, in order to increase the density, the duration of the transfer voltage is increased relative to the duration of the reverse transfer voltage, because such an adjustment increases the amount by which developer adheres to the image bearing member, increasing thereby the density.
However, in both types of methods, the transfer voltage and reversal contrast (difference between the potential level of the non-image portion of the electrostatic latent image on the photoconductive member, and the potential level of the DC component of the development bias) increase in magnitude in the high density range and low density range, resulting in background fog and/or reversal fog, which are unignorable problems in some cases.
More specifically, an attempt to increase the image density to a level higher than a predetermined level increases the magnitude of the transfer voltage, which causes developer to adhere to not only the image portions but also non-image portions, resulting in increases in the so-called background fog. On the other hand, an attempt to reduce the image density to a level below a predetermined level increases the magnitude of the reversal contrast (difference in potential level between the reverse transfer voltage and the dark portions of the photoconductive member), by which the developer particles, which are inherently chargeable to the negative polarity, and yet have been charged to the positive polarity, are substantially affected, increasing the amount by which the reversal fog is produced.
In the case of the method for increasing the density of the developer image by increasing the ratio of the duration of the transfer voltage relative to the duration of the reverse transfer voltage, instead of changing in magnitude the transfer and/or reverse transfer voltages of the bias voltage, the amount of the background fog tends to increase due to the increase in the duration of the transfer voltage.
Thus, a method for reducing the background fog has been proposed. More specifically, a method for increasing the density of a developer image by increasing the ratio of the duration of the transfer voltage relative to the duration of the reverse transfer voltage, while reducing the magnitude of the development contrast (difference in potential level between the transfer voltage and the image portions of the electrostatic latent image), has been proposed in Japanese Laid-open Patent Application 2000-98710. However, from the standpoint of increasing the image density while preventing the occurrence of the background fog, this patent application is desired to be further improved.
The primary object of the present invention is to provide a developing apparatus capable of preventing the occurrence of such fog that tends to occur when increasing the density of a developer image.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.